Speaking as chairman of the committee, Sen. Herb Kohl (D-Wis.) said in a statement: “We believe the elimination of competition between XM and Sirius is contrary to antitrust law and the interests of consumers. We urge that the FCC find the merger contrary to the public interest and exercise its authority to block it."

Saying that he was disappointed with the DOJ's decision, he went even further, adding: "We are particularly disturbed by this decision given the Justice Department's record in recent years of failing to oppose numerous mergers that reduced competition in key industries, resulting in the Justice Department not bringing a single contested merger case in nearly four years."

DOJ assistant attorney general Thomas Barnett took issue with the senator's characterization.

"The antitrust division has been very vigilant in enforcing the antitrust laws," he told reporters Monday. "Just this year, we had 10 mergers that we have challenged … I am quite comfortable that we are vigorously but appropriately enforcing the antitrust laws with respect to mergers."